Measurement of soluble circulating fibrin is possible using a technique in which the fibrin stabilizing enzyme, Factor XIII, enzymatically incorporates radioactive C14 glycine ethyl ester into the soluble circulating fibrin. The technique is sensitive to thrombin alteration of fibrinogen (soluble circulating fibrin) but insensitive to plasmin alteration of fibrinogen and fibrin (fibrinogen and fibrin split products). Four areas for further investigation are proposed. Measures to improve the rapidity with which the assay can be done as well as its sensitivity at low fibrinogen concentrations will be investigated. These measures will include the used of an insoluble antifibrinogen antibody for recovery of fibrinogen and labeling the soluble circulating fibrin in unfractionated plasma. Experiments using SDS acrylimide gel electrophoresis to localize the sites of labeling in soluble circulating fibrin and normal fibrinogen will be done. These experiments may lead to determining whether thrombin altered fibrinogen is present in normal plasma. Studies of the half life of soluble circulating fibrin in experimental animals will be pursued. Further clinical experiments to document intravascular coagulation in newborn infants, in patients with renal transplant rejection, sepsis, and thrombo-embolic diseases will be designed.